Hydrodynamic volatile escape and moderately volatile isotope fractionation in giant impact ejecta

The Moon and bulk silicate Earth (BSE) are compositionally similar across multiple bulk compositional and isotopic domains, suggesting a common material origin. However, the Moon is strongly depleted in volatile elements and fractionated in heavy moderately volatile element (MVE) isotopes relative to Earth, which is often interpreted to be the result of partial vaporization of the protolunar materials occurring sometime between the giant impact and the crystallization of the lunar magma ocean. Here, we model devolatilization and MVE isotope fractionation due to impact vaporization and partial hydrodynamic escape of a vapor jet in the immediate aftermath of the Earth and the impactor, Theia by combining impact simulations with the smoothed particle hydrodynamics (SPH), 1D escape model, and phase calculations with MAGMA. We investigate the canonical Moon-formation model and half-Earths models for this work. We find that partial hydrodynamic escape of the vapor jet and total loss of the non-escaping portion of the impact vapor from the disk can remove up to ∼20% of the total volatile budget, falling short of the observed ∼75−99% depletion of lunar volatiles including K, Na, and Zn relative to Earth. We also find that the canonical model can remove more volatiles than the half-Earths model due to its geometry. Impact vaporization can explain the observed lunar heavy 41K enrichment relative to the Earth (Δ41K) under a partially saturation condition for both models. Here we propose that the MVE loss may have occurred by two steps; (1) impact-induced volatile loss, and (2) additional loss from the lunar magma ocean or from the Moon-forming disk, which relaxes prerequisite of the models for the secondary loss. Additionally, we also leave the composition of Theia as a free parameter and find that a volatile-poor Theia cannot impart the Moon's volatile depletion. We use Mg/Si vs Al/Si mass ratios to further speculate on the origin of Theia, finding that Theia resembles ordinary or carbonaceous chondrites and that an enstatite-like Theia is only possible if the Theian core was Si-rich.